44,716 research outputs found
Static Spherically Symmetric Kerr-Schild Metrics and Implications for the Classical Double Copy
We discuss the physical interpretation of stress-energy tensors that source
static spherically symmetric Kerr-Schild metrics. We find that the sources of
such metrics with no curvature singularities or horizons do not simultaneously
satisfy the weak and strong energy conditions. Sensible stress-energy tensors
usually satisfy both of them. Under most circumstances these sources are not
perfect fluids and contain shear stresses. We show that for these systems the
classical double copy associates the electric charge density to the Komar
energy density. In addition, we demonstrate that the stress-energy tensors are
determined by the electric charge density and their conservation equations.Comment: 11 page
Monopole decay in the external electric field
The possibility of the magnetic monopole decay in the constant electric field
is investigated and the exponential factor in the probability is obtained.
Corrections due to Coulomb interaction are calculated. The relation between
masses of particles for the process to exist is obtained.Comment: 13 pages, 8 figure
Quantum phase transitions in the quasi-periodic kicked rotor
We present a microscopic theory of transport in quasi-periodically driven
environments (`kicked rotors'), as realized in recent atom optic experiments.
We find that the behavior of these systems depends sensitively on the value of
Planck's constant : for irrational values of they
fall into the universality class of disordered electronic systems and we derive
the microscopic theory of the ensuing localization phenomena. In contrast, for
rational values the rotor-Anderson insulator acquires an infinite (static)
conductivity and turns into a `super-metal'. Signatures of the corresponding
metal/super-metal transition are discussed.Comment: 4 pages, 1 figure, 1 tabl
On a discretization of confocal quadrics. I. An integrable systems approach
Confocal quadrics lie at the heart of the system of confocal coordinates
(also called elliptic coordinates, after Jacobi). We suggest a discretization
which respects two crucial properties of confocal coordinates: separability and
all two-dimensional coordinate subnets being isothermic surfaces (that is,
allowing a conformal parametrization along curvature lines, or, equivalently,
supporting orthogonal Koenigs nets). Our construction is based on an integrable
discretization of the Euler-Poisson-Darboux equation and leads to discrete nets
with the separability property, with all two-dimensional subnets being Koenigs
nets, and with an additional novel discrete analog of the orthogonality
property. The coordinate functions of our discrete nets are given explicitly in
terms of gamma functions.Comment: 37 pp., 9 figures. V2 is a completely reworked and extended version,
with a lot of new materia
Loop-Induced Stochastic Bias at Small Wavevectors
Primordial non-Gaussianities enhanced at small wavevectors can induce a power
spectrum of the galaxy overdensity that differs greatly from that of the matter
overdensity at large length scales. In previous work, it was shown that
"squeezed" three-point and "collapsed" four-point functions of the curvature
perturbation can generate these non-Gaussianities and give rise to
so-called scale-dependent and stochastic bias in the galaxy overdensity power
spectrum. We explore a third way to generate non-Gaussianities enhanced at
small wavevectors: the infrared behavior of quantum loop contributions to the
four-point correlations of . We show that these loop effects lead to
stochastic bias, which can be observable in the context of quasi-single field
inflation.Comment: 10 pages, 4 figure
Majorana Fermions in Strongly Interacting Helical Liquids
Majorana fermions were proposed to occur at edges and interfaces of gapped
one-dimensional systems where phases with different topological character meet
due to an interplay of spin-orbit coupling, proximity-induced superconductivity
and external magnetic fields. Here we investigate the effect of strong particle
interactions, and show that the helical liquid offers a mechanism that protects
the very existence of Majorana edge states: whereas moderate interactions close
the proximity gap which supports the edge states, in helical liquids the gap
re-opens due to two-particle processes. However, gapless fermionic excitations
occur at spatial proximity to the Majorana states at interfaces and may
jeopardize their long term Majorana coherence.Comment: 7 pages, 4 figure
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